of connebsville



\ l646,3 Oct. 18,1927. WSILKIN 73 HIGH SPEED LOBED IMPELLER BLOWEROriginal Filed April 14, 1926 3 Sheets-Sheet 1 l INVEN'TOR.

' John T. Wzlkm;

' ATTORNEYS v 7 1,646,373 J. T. WlLKlN HIGH SPEED LOBED IMPELLER BLDWEROct 18, 1927.

v Original Filed April 14, 1926 3 Sheets-Sheet 2 I INVENTOR c/b/m T.Wz'lkz'n,

ATTORNEYS Patented a. 18, 1927.

UNITED STATES PATENT OFFICE.

JOHN T. WILKIN, OF CONNERSVILLE, INDIANA, ASSIGNOR TO THE CONNERSVILLEBLOWER COMPANY, OF CONNERSVIIJLE, INDIANA, A CORPORATION OF INDIANA.

HIGH-SPEED LOBED-IMPELLER BLOWER.

Substitute for application Serial No. 101,903, filed Apri114, 1926.

- Serial No. 171,913.

In that type of rotary gas pumps and blowers primarily designed forincreasing or decreasing gaseous pressures, and wherein the fluid flowis caused by the action of two interdigitated rotary lobed impellers arranged within a casing, the two impeller elements have mating surfacesof such a character as to form gas forwarding pockets between theimpellers and the casing and also to maintain minimum and uniformclearance between the impellers themselves and between each impellerandthe casing. Such impellers do not coact for relative rotation but arerotatively connected by intermeshing timing gears which also serve totransmit power from one impeller shaft to the other. 1

For convenience of description I shall designate such a structure as ablower,

whether it be designed or used as a blower or gas pump, because thecoaction of the several elementsis the same in either case.

In the early period of development of blowers of this type their use waslargely smelting furnaces, forge fires and theoperation of cash carryingtubes for store service work; also for gas exhausters in gas works, allof which uses required pressure differences rarely exceeding one or twopounds per square inch. The demand for struc tures of this type hasgradually increased until now they are used for many purposes whichrequire pressure differences as high as fifteen pounds. For convenienceof description I shall. use the term low pres sure for pressuredifferences below two and a half pounds, and the term high pressure forpressure differences between two and a. half and fifteen pounds.

Heretofore in the manufacture of blowers of this type it has been thepractice to make the lobes of the impellers hollow and as light as isconsistent with necessary rigidity.

I have found by long experience with blowers of this type having lightimpellers, each with two hollow lobes, that where the driving power isapplied to one shaft, and the other impeller is driven through thegears, the wear on. the teeth of the gears in limited to the blowing offoundry cupolas,

This application filed March 2, 1927.

not uniform in all parts of the revolution, but is concentrated on a fewteeth at two' opposite portions of the revolution.

It has long been known, in the operation of blowers of this type, whereeach impeller has two. lobes, that there is, twice in. each cylicangular distortion or twist of eac shaft between the connecting gearsand the impellers, and that this distortion or twist permits a change inthe angular relation of the impellers to each other. The amount of thisangular distortion increases directly with the increase of theunbalanced area of each impeller and the pressure actin there-Theoretically the mating sur aces of the impellers should just avoidcontact, and the actual clearances between such mating surfaces should,of course, be made as small as possible in order to decrease slippage orleakage of the gas from the high pressureside to the low. pressure sidebetween the mating lines of the two impellers.

In blowers of this type the highly fluctuating torque and unavoidedinaccuracies of gearshave necessitated excessive clearances betweentheimpellers, andhave limited the practicable speed to a pitch linevelocity of approximately 700-feet per minute in small machines andapproximately 1400 feet per minute in large machines, more or lessdepending on workmanship, materials, etc., because when higher speedswere attempted there would be a noisy and destructive action of thegears and impellers unless further and more excessive clearances weremaintain continuous contact of the working faces of the gear teeth, andthat this failure due 'to inaccuracies of commercial produc tion, whichgive fluctuating impulses to the driven gear, such impulses, as iswell-known, increasing in intensity as the square of the increasedspeed,p 7

I have alsoff'oun'd that, in such blowers this 'destructive an'd noisyaction increases as the gearsbecome worn, because, as previou-slystated, the wearof the gear teeth, when'light impellers are used, is notuni- 7 form throughout -the circumference but is concentrated on'a fewteeth'at two opPO gen-ions of the gear. It is, therefore, evient itlratthisfuneven wear of the gears woiild tend to transmitto the drivenimpeller additional cyclic impulses. 7 one object of my invention is toso construct a blower or gas pump ofthe type described that *theimpellers maybe commerciallysuccessfully driven at very much higherspeeds than has heretofore been found -'practicable and'to commerciallysuccessfully drive such impellers at such in til creased speeds -againsthigher pressures than'have heretofore been found practicable at-suchhighspeeds, the construction beings'u'ch "that necessary clearancesbetween the impellers may be substantially decreased.

"Terms end my invention contemplates the provision of impeller elementsof a and sufficient flywheel effect (most conveniently obtained bysubstantially increa'sing the, weight of the impellers which 'in'thehigher pressure'machincs having relatively short impellers would besolid or nearly solid) coordinated with and'proportioiiailtothe variabletorque exerted on'the impellers by the gas forces, thus decreasingobjectionable torsional angular displacements" of the impellers withrelationto each other" and avoiding torsional reactions of theshafts'thereby preventing breaking of ccntact of the driving faces ofthe gear teeth by such reactions; and further by providing gears withteetlrwhich willnot impart to the driven gear suchfiuctuatingimpaissae-wnrcause the breakingof contacts of the driving faces of theteeth.

Bythes'e'means I eliminate, or practically eliminate the noise anddestructive impacts of the gear teeth and impellers at speeds muchhigher than has heretofore heenfound possible, namely at speeds from2000 to 5000 (and above) feet per minute pitch line velocity, thusmaking possible the utilization of smaller, and more efficient units atlower cost,

In other words, I have discovered that if the impellers themselves bemade sufficiently heavy, so as to give them a very much greater inertiavalue than they have heretofore possessed, the cyclic distortion of theshafts between the impellers and the gears, particularly in thatimpeller which is driven through the gears, will be decreased and thatconsequently a blower in which the impellers themselves have asufficientinertia valuemay be driven at speeds very much in excess ofpresent practices, while at the same time the clearances between suchimpellers may be substantially reduced, and more uniform wear of thegears will be attained.

In orderto make the matter clear:

impeller and'the impeller which is rotated through the medium of thegears may be called the driven "impeller. 'When the driven impeller hasthetip of one of its lobes in immediate association with thewaist'of thedriving impellenthe pressure on the gear teeth and the resistance torotation offered by the driven gear, due tothe pressure in the pressureside of the system,

one impeller, to the shaft of which the driving .power is applied, maybe called the driving are at their minimum. As this drive'nim v pelleradvances the gear pressure and e'ffective resistance to forward movementof the driven impeller increase and, as this (:lrivenimpeller is beingforwardedby the gear connection, the shaft of the driven im peller,between the impeller and the a gear is twisted. lVhen the drivenimpeller 'reachesa point where its waist is in association with the tipof the driving impeller, the gear pressure andresistance'to forwardmovement of the driven impeller are at their maximum.

Immediately thereafterthe advanced tip of the driven impeller leavesassociation with the casing and the pressure in the pressure side of'thesystem enters the on-coming delivery pocket between the impeller and thecasing thus causing a sudden pressure equalization, and the resistanceto forward movement of'the driven impeller decreases until the advancingtip-of the driven inipeller is in association with the'waist of thedriving impeller, at which time the gear pressure and resistance toforward move. ment of the driven impeller have returned to minimum.During this period of decreasing impeller torque, the twist of the shaftof the driven impeller between that impeller and its gear has beencorrespondingly decreased, such decrease taking effect quite promptlyupon the-entry of the pres. A

sure into the on-coming delivery pocket justmentioned, and the lighterthe impeller, the more promptly its shaft twist .will be released. Therelease of this shaft twist causes, or has a tendency to cause, thedriven impeller to accelerate because of the energy delivered by theuntwisting shaft, and this acceleration causes, or tends to cause, aseparation of the driving connection between the gear teeth.

During the above described period. ofaction of the driven impeller andits shaft, a similar action is occurring in the driving impeller and itsshaft, but, because of the cycles of the two being spaced ninety degreesapart, the combined effect is to produce twice per revolution anincrease of pressure on the few teeth trasmitting maximum power to thedriven impeller, and a subsequent increased tendency to cause aseparation of the driving connection between the gear teeth at the timeof minimum transmission of power to the driven impeller.

It will now be recognized that the smaller the inertia value of theimpellers, the greater will be the acceleration of the driven impelleras the crest of resistance to forward movement is passed, and thatconsequently an increase in the inertia value of the impellers willdecrease the amplitude of their torsional fluctuations. \Vhen this isrealized it becomes apparent that, because inertia varies directly asthe mass and as the square of the velocity, the necessary inertia valueto substantially eliminate objectionable torsional fluctuations of theimpellers and produce more uniform gear wear, maybe obtainedby'sufficient increase in mass and speed and that such increase of speedbecomes possible because the inertia value of the impeller has been madesufliclent to mam- ,tain. practically uniform angular velocity of theimpellers and their tion toeach other.

According to recognized practice prior to my present discovery, blowersof this type with impellers having two hollow lobes have proper angularrelagenerally been designed with standard shafts and bearings for eachgear diameter so that the standard elements, i. e.,impellers, shafts,bearings, gear teeth, etc., were capable of transmitting a given normalH. P. per revolution. Also the lengths of the impellers have been madeto vary inversely as the pressure against which the blower operates.That is to say, low pressure operation permitted use of axially longimpellers and, as desired pressures were greater, these impellers wereaxially shortened in a mannor to maintain approximately the same H. P.per revolution, the same total pressures on impellers, bearings, gears,etc.

Under that practice the shortening of the impellers for highpressurework reduced their mass and thus reduced thier inertia value, whereas,according to my invention, the mass of the shorter impellers, relativeto their length, is increased and their inertia value is maintained orincreased. Since, according. to my present discovery, the shorterimpellers for higher pressures are very substantially heavier thanheretofore,

their inertia value will be sufficient to practically eliminate theobjectionable cyclic acceleration of the driven impeller and prevent thebreaking of driving contact of the gears, and this in itself permitsadditional speed which, in turn increases the inertia value of theimpellers.

When a sufficient mass is provided, proportionate to the pressureagainst which the blower is to work, the impellers should, of course, bespeeded up, in order to gain the advantage of my invention.

The design should be such therefore, that desired volume and pressure ofdelivery being known, impeller'proportions should be determined on thebasis of a large inertia value of the impeller, such inertia value to bea function of speed and mass, the mass to be relatively large andsufficient at the adopted speed to prevent objectionable relative cyclicangular fluctuations of the impellers.

Methods of calculatingthe variation of angular velocity of massessubjected to fluctuating torque, can befound in engineers hand books andtext books dealing with the v design of flywheels for reciprocatingsteam engines.

There is another distinctly beneficial result attained by the use ofhigh inertia impellers which deserves consideration. Just as each gasforwarding pocket is opened to the delivery, or high pressure, side ofthe system, there is a sudden increase of pressure in that pocket andthis increase of pressure, acting upon the entire side area of theimpeller, delivers to the impeller, transversely of its axis and towardsthe mating impeller, a heavy blow which, if the impeller be light, ashas heretofore been the practice, actually displaces the impeller andits shaft sidewise, the extent of this lateral displacement beingdetermined by the looseness of the bearings, the flexibility of thebearing supports and the flexibility of the impellers and shafts. Itnow, the shorter impellers for higher pressures be made solid or nearlysolid, and the longer impellers be made adequately heavy, theirincreased inertia value is not only effective as a resistance tofluctuations in angular velocity but also has beneficial. effect asresistance to the above mentioned, lateral displacements caused by thesudden blow of pressure entering the oncoming pocket. This lateraldisplacement of the impeller and its shaft is ,my invention.

tionof a blower or gas pump intended to 7 very objectionable, because itmakes it almost impossible'to maintain tight packing around the shaft.where sucha structure is used as a gas .pump and where leakage aroundthe shaft is objectionable. This lateral deflection of the shaft alsoproduces uneven side wear of the, journal bearings thus changing thedistance between the gear centers and causing irregular and'noisyaction-of thegears.

The accompanying drawings illustrate Fig. 1 isa transverseseccomputation diagram for detern'iining a de-V .sirable gear toothform, and Fig. 6 an adiarbatlc compression diagram.

In the drawings 10 indicatesthe casing, 11 and 112 the impeller shafts,13 and 14: the lobed impellers interdigitated with each other, 15 theinlet, 16 the outlet and 20, 21

the connecting gears- With the exception of the above described inertiavalue of the impellers and reduced clearance between the impellers,theseele- :ments are of the general form commonly known in the art andthe particular contours-of the impellers may be considerably varied inaccordance with well-known practice. The relatively short impellers,however,are shown as solid because, in the practice of my invention,these impellers must'haveoa. very high inertia value and the mostconvenient way to obtain such high inertia value is to make their lobessolid or nearlysolid.

I am :awarethat liquid pumps of the type under consideration have beenmade with. impellers having solid lobes, but the speed of suchpumps wastoo low to develop the inertia values of the impellers as a functionofihi'gher speed of operation.

'My-invenilion does not apply to pumps of this type designed for pumpingliquids, because-operation of such pumps at high speeds becomesdestructive to the pump. due

to the weight and incompressibility of the liquids.

The increase in compression efiiciency by high speed operation of ablower or gas pump of the'type under consideration .becomes apparentfrom a study of Fig. 6, which shows'the relations of pressures andvolumes and the percentage of the discharge strokeire uiredto. compressthe gases to specified disc arge pressures. It will be noted that thehigher the pressure againstwhi'ch the bloweris-operating, the greaterthe percentage of" the stroke required to produce the necessarycompression. In the operation the gas which is being transferredfrom alower livery pocket at an early point in the com; pression period andthe impellers must,

therefore, make their advance against the full discharge pressure.- y

It'is evident from a study of these relations that the higher thepressure against which'the blower is operating,the higher should be thespeed in order to take advantage of the inertia of the gas andaccomplish the compression of each dischargevolume before the gas in thedischarge linehas time to overcome its inertia and expand backward intothe oncoming discharge pocket. By such action the compression efliciencyOfgthfi blower t higher pressures is very muchincreased. Taking forexample a blower operating against 10- lbs. pressure, it will be notedthat the mean effective adiabatic pressure when compressing to 10 lbs.is 8.27

lbs. If a slow running blower is working against 10 lbs. pressure,the.mean effective pressure against the impellers will at all times closelyapproximatelO lbs, butiftthe blower is so designed and speeded as toaccomplish approximately I adiabatic compression, the mean effectivepressure'would only be 8.27 lbs, and a consequent savingin horsepower of17.3 per cent.

shorter leakage lines and smaller clearances.

if that smaller blower maybe run at a higher speed than has heretoforebeenfound practicable.

I have found that, inzconnection with providingthe impellers of highinertia =value, some further advantages may be obtained by providingconnecting "gears having teeth which are fiexible'to an extent equahxorsubstantially equal to the unavoidedidepartures from theoreticalaccuracy'of'the'gears existing in tooth form, toothspacing and mounting.

Another object of my invention tetherefore, to combine connecting gears;having suitably flexible teeth, with the high-inertia hi h-speedimpellers.

Where the teeth have been produced by commercially practicable methodsthere are unavoidable inaccuracies from theoretical perfection and theseinaccuracies vary in individual teeth. At the point Where two teethfirst come into action, the power-deliverytooth is contacted at apointslightly below the pitch line, while the power-receiving tooth 1scontacted at, or approximately at, the top of the tooth which is, ofcourse, much further beyond the pitch line than is the point of contactbelow the pitch line on the power-delivery tooth. Bearing in'mind thefact that the shapes of the teeth should be such that, when in contactat the pitch line there should be a true rolling contact, it becomesapparent for several reasons that, in order to avoid hammering atthemoment of initial contacting oftwo teeth, thepower receiving tooth isthe one which at the point of initial contact, should have greater yieldthan thepower-delivery tooth.

For instance, if the powerreceiving tooth is incapable of suflicientyield, then one of two things must take place. Either thepower-receiving element must, as a whole, be suddenly accelerated, orthe power=delivery element must, as a whole, be suddenly retarded. It isapparent, therefore, that the preferable construction, by which individual yield of a tooth may be obtained, should be such that the greatestyield shall take place in the tooth to which the driving force is beingapplied, and this greatest yield should take place atv the timeofinitial con tacting i. e., when the tooth is coming into action.

It is also desirable that whatever of yield there be in each tooth willtake place under such conditions as to avoid as much as possibletendencies'towards crystallization at any particular section. i

The epicycloidal type of tooth, having a radial flank below the-pitchline is, of course that type of tooth which in itself is capable ofgreater yield, but the weakest point of this tooth is 'at'its narrowestcross-section which is at the root, and this type of tooth is thereforeincapable, without modification, of satisfactorily providing the yieldwhich is necessary for the practice of my invention, without danger ofcrystallization and breaka e atthe root.

Bearing in mind that the tooth may be considered as receiving its loadon lines nor mal to its surface, and that at the time when aload-receiving tooth comes into action the load may be consideredasbeing applied along the line LL, (Fig. 5) thetooth may be analyzed asbeam of uniform flexure (one having a parabolic section) fixed at oneend and having its load applied at its outer end T. Applying, then the.;usual analysis for such a beam loaded at the point T and having, atthe root line, a thickness AB equal to the normal root thickness of an0rmal epicycloidal tooth, it is apparent that, that tooth may be givenan additional resilience theoretically capable of uniform stressthroughout its length by forming a sub-root with sides approximating theparabola secof the tooth defined by the points ABCD may of course bevaried,idepending upon the metal used, the amount of velocity consideredto be necessary, the inertia value of the impellers and various otherfactors whichwill readily suggest themselves to a competent designer,and dependent upon circumferential velocities desired, etc, being in allcases sufficient to insure a quiet openation of the gears.

.At first glance it might be supposed that the tooth at the points ABwould have a weak cross-section, but. upon further consideration itbecomes apparent that that would not be the case because the parabolaMAC or MBD is the uniform stress line and therefore the additional metalin the tooth which lies between the exterior of the tooth andthe-parabola section MA or MB is simply additional strength or wearingmaterial and there are no lines of weakness at the points A and B. i

In order that the method of computation may be understood I giveherewith. the

mathematical analysis for a specific example:

a 1 Gear specifications. Pitch diameter 26". Face ll. Speed=400 R. P.No. teeth=37. Three teeth to be always in contact. H. P. to betransmitted=346fi H. P.

Velocity pitch line= 400 X 26 X 3.1416 2722.7 ft. per min.

t 12 a U r .547 X 35000 7 LQfldPGI'llOOth'Q 2W =1,4 ()0# (app.)-

If the load is borne by one tooth g 1 ==382i$per 1 face. Teeth to beepicycloidal radial flankform,

51.- ini' oluteofsulficient tug tandem? to "Now iii order a) find thethickness (21) I permit deflection as desired.

I thepoint of initial'conta'ct at T (Fig:- 5); the tooth pressure isexerted along the hneof' action, cutting'the center line of tooth atM'.The distance 0.7 5'T isselected' as'a inimum safe. flank-length whichwill" allow proper tooth" action Without interfer ence, and such aflank-length according tofl root accepted rules of design, requires athickness, at AB, 0151.040.-

Gircular pitch'=2. 2076:

i Thickness of tooth 1.1038;

It parabolic curve'isnow generated with the point M as the vertexand-the distaneeAb as adouble ordinate, it' willireprecent-i'leverbeamof uniform strength havingtheloa concentrated at: M. Allof the toothoutside of the parabolic curves wil1;,be excess-material, Iftheparabo'lie curves; and- MYB are. continued: as, shown} on; thedrawing to G7 andv D, then the portion,.A iB C-D-Willbea portion ofuniform strength. fI'n other ords, the fiberv it apprioaches astraight'line so that for all practical purposes the sides of: the toothalong: AG-and; 13D may be made'straight.

The mathematical calculations used to findi thethickness (d) of thesub-root A, whiohis a double ordinate of the parabola,

' at: any: place-within-the portion of uniformstnength is follows.

First calculate the maximum; fiber stress i SQQL BL); of the section AB.

M=bending moment.

C= thickness of tooth at-section under consideration. i v I=moment ofinertia of tooth at section a under consideration. M=382 1.84L=7O2.88

It should be noted that have selected the Worst condition possible byconsidering a the entire'load of per'l of itace to be borne by the'oneimperfect toot-hat its point of initial. contact. Thisgives- S(AB) amaximum value.

at any other distance (L) from the Vertex: We must keep the maximumfiber stress s constant at 3900# and solvefor in: the equation+ andthusgenerate the arabolic curves,

The values of d and" however shouldbej such as to give'the desiredamount of, de-

flection. I v I v Now consider that We desire a' deflection of 0.00108in the; tooth taking the tooth as:

a straight cantilever beam with the load concentrated the free end t 3then Itis thus seen" that W6 5 can find Values: of

By giVingL'difierent valueswe may solve any number ofcorrespondingvalues for d E uaat (im- 0 beam of uniform strength andload acting on the pitch line.

It should be noted that in solving for and L any deflection formula maybe used. Also the figures and methods which are given are to act as anillustration only, and do not limit the design or construction of thetooth which is essentially a gear tooth having a portion of uniformflexure.

This application is filed as a substitute for application No. 101,903,filed by me April 14th, 1926.

I claim as my invention:

1. A blower comprising a casing, two interdigitated lobed impellersmounted with in the casing, and a pair of meshing gears rotativelyconnecting said impellers, the said gears having teeth capable ofcircumferential yield under the applied load of approximately theunavoided departure of the contacting surfaces of said teeth fromtheoretical accuracy.

2. A high speed blower comprising a casing, two interdigitated lobedimpellers mounted within the casing and having an inertia value which ishigh relative to their gas load torque variations, and a pair of meshingmetal gears rotatively connecting said impellers, the said gears havingteeth capable of circumferential yield under applied load ofapproximately the unavoided departure of the contacting surfaces of saidteeth from theoretical accuracy, whereby the tendency of tooth wear istoward uniformity throughout the circumferences of the gears.

3. A blower comprising a casing, two interdigitated lobed impellersmounted within the casing and a pair of meshing gears rotativelyconnecting said impellers, the said gears having teeth capable ofcircumferential yield under applied load of approximately the unavoideddeparture of the wearing surfaces of said teeth from theoreticalaccuracy, each of said teeth having a subroot of approximately uniformflexure,

4. A high speed blower comprising a casing, two interdigitated lobedimpellers mounted within the casing and having an inertia value which ishigh relative to their gas load torque variations, and a pair of meshingmetal gears rotatively connecting said impellers, the said gears havingteeth having a sub-root ofapproximately uniform flexure and capable ofcircumferential yield under applied load of approximately the unavoideddeparture of the wearing surfaces of said teeth from theoreticalaccuracy, whereby the tendency of tooth wear is toward uniformitythroughout the circumferences of the gears.

75. A high speed blower comprising a casing, two interdigitated lobedimpellers mounted within the casing, and a pair of meshing metal gearsrotatively connecting the said impellers and having teeth of a formproviding at least two pairs of teeth in mesh at all times and capableof circumferential yield at the contacts, the inertia value of saidimpellers being relat-ivelyhigh, whereby the tendency of tooth wear istoward uniformity throughout the circumference of the gears.

6. A high speedblower characterized by a casing, two interdigitatedlobed impellers rotatably mounted within and coacting with the casing,and a pair of meshing gears rotatively connecting said impellers, thesaid impellers having an inertia value which is suflicient to maintaincontinuous contact of the working faces of the gear teeth at high speedsand which is high relative to their gas load torque variation, wherebythey may be rotated at high speed with small clearance.

7 A high speed blower characterized by a casing, two interdigitatedlobed impellers rotatably mounted within and co-acting with the casing,a pair of meshing gears rotatively connecting said impellers, saidimpellers having such great mass as is adequate, at the intended highspeed of rotation, to develop inertia value which is sufficiently highrelative to the cyclic gas load torque fluctuation to maintaincontinuous contact of the working faces of the gear teeth at suchspeeds, whereby small clearance between the impellers may be employedand eflicient, normal, high-pressure, highspeed operation assured.

8. A high speed blower comprising a casing, two interdigitated lobedimpellers mounted within the casing, and a pair of gears rotativelyconnecting said impellers and having teeth of a form providing at leasttwo pairs of teeth in mesh at all times, the inertia value of saidimpellers being high relative to their gas load torque variations,whereby the tendency of tooth wear is towarduniformity throughout thecircumference of the gears.

In testimony whereof, I have hereunto set my hand at Connersville,Indiana, this 24th day of February, A. D. one thousand nine hundred andtwenty-seven.

JOHN T. WILKIN.

